Advanced Laser Systems for MEGa-ray-based Nuclear Materials Detection and Assay

摘要

A Compton-scattering based, tunable MonoEnergetic Gamma-ray (MEGa-ray) source suitable for nuclear resonance fluorescence measurements requires specific performance parameters for the affiliated laser systems. Optimization of the number of photons/eV/s dictates a trade-off between shorter laser pulses to maximize the photon density, and narrower laser bandwidths to minimize the gamma-ray energy spread. Also required is a high-brightness electron beam, which in turn requires a second laser system, converted to UV, with a fast rise-time, long duration, and flat transverse profile. Furthermore, these lasers must be synchronized with each other and with the RF providing the electron bunch acceleration. Presented here is an overview of the laser system designed for LLNL's MEGa-ray source. This chirped-pulse-amplification (CPA)-based laser system starts with a fiber-based oscillator with is then split into two amplification chains. The first amplification chain produces 120 Hz, 1 mJ, 250 fs, 1053 nm laser pulses in a series of fiber amplifiers. This pulse is then frequency-quadupled to the UV, shaped spatially and stacked temporally to produce the desired laser distribution for the photocathode. The second amp chain generates a 120 Hz, 1 J, 10 ps, 1064 nm laser pulse using a combination of fiber amps and diode-pumped Nd:YAG heads. A novel hyper-dispersion stretcher/compressor pair allows CPA to work effectively with the narrow bandwidth of the gain medium. Finally, in order to increase the laser-to-gamma-ray conversion efficiency, the laser photons can be recirculated through the interaction point via an optical cavity that traps pulses using a frequency conversion process (a scheme known as "RING": Recirculation Injection via Nonlinear Gating).